Objective To assess the effect on the human sex ratio at birth by assisted reproductive technology (ART) procedures.Design Retrospective population-based study.Setting Fertility clinics in Australia and New Zealand.Population The study included 13 368 babies by 13 165 women who had a single embryo transfer (SET) between 2002 and 2006.Methods Logistic regression was used to model the effect on the sex ratio at birth of ART characteristics [in vitro fertilisation (IVF) or intracytoplasmic sperm insemination (ICSI) SET, cleavage-stage or blastocyst SET, and fresh or thawed SET] and biological characteristics (woman's and partner's age and cause of infertility).Main outcome measures Proportion of male births. ResultsThe crude sex ratio at birth was 51.3%. Individual ART procedures had a significant effect on the sex ratio at birth. More males were born following IVF SET (53.0%) than ICSI SET (50.0%), and following blastocyst SET (54.1%) than cleavage-stage SET (49.9%). For a specific ART regimen, IVF blastocyst SET produced more males (56.1%) and ICSI cleavage-stage SET produced fewer males (48.7%).Conclusions The change in the sex ratio at birth of SET babies is associated with the ART regimen. The mechanism of these effects remains unclear. Fertility clinics and patients should be aware of the bias in the sex ratio at birth when using ART procedures.Keywords ART births, blastocyst, cleavage-stage embryo, ICSI, IVF, logistic regression, sex ratio at birth, single embryo transfer.Please cite this paper as: Dean J, Chapman M, Sullivan E. The effect on human sex ratio at birth by assisted reproductive technology (ART) procedures -an assessment of babies born following single embryo

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